Set the EDXAPP_IMPORT_EXPORT_BUCKET setting to an empty string

[replaceafill]

This PR makes the course import/export functionality to use the file system for storage.

@jibsheet As requested on https://github.com/edx/edx-platform/pull/15005#issuecomment-305627209

Configuration Pull Request

Make sure that the following steps are done before merging:

• A DevOps team member has approved the PR.
• Are you adding any new default values that need to be overridden when this change goes live? If so:
  • Update the appropriate internal repo (be sure to update for all our environments)
  • If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.
  • Add an entry to the CHANGELOG.
• Have you performed the proper testing specified on the Ops Ansible Testing Checklist?

[openedx-webhooks]

Thanks for the pull request, @replaceafill! It looks like you're a member of a company that does contract work for edX. If you're doing this work as part of a paid contract with edX, you should talk to edX about who will review this pull request. If this work is not part of a paid contract with edX, then you should ensure that there is an OSPR issue to track this work in JIRA, so that we don't lose track of your pull request.

Create an OSPR issue for this pull request.

[jibsheet]

Building a sandbox to test this